Molding composition and method of making same



United States Patent EdwardLund, La Crosse, Wis., assignor to Allis-Chalmers Manufacturing Company, Milwaukee, Wis.

No Drawing. Application August 27, 1954, Serial No. 452,719;

2 Claims. c1. roe-sass This invention relates to the casting of metals in sand molds and is more particularly concerned with a molding V composition for use as a facing for sand molds.

In the foundry art there are many methods for preparing molds from sand, but generally the methods maybe classified in two groups, namely, green sand molding and dry sand molding. In green sand molding, the mold is formed of sand containing varying amounts of moisture and. the molten metal is poured into the mold so formed. In dry sand molding, on the other hand, the mold is formed from sand and the excess moisture in the sand is removed from the mold by curing or drying it prior to the time of pouring.

Dry sand molds are usually considered indispensable in making castings which must be smooth and clean and in which intricate and minute details of the pattern are to be reproduced with great accuracy. Unfortunately, dry sand molds are expensive to make because of the great amount of equipment and labor required to cure the sand molds and because of the long period of time required to prepare each mold for pouring.

Although green sand molds are relatively inexpensive to use, their moisture content creates problems which limit their field of application. Upon contact of molten metal with the mold surfaces, the moisture turns to steam causing the mold to expand so that the dimensions of the finished casting may be inaccurate. Furthermore, in escaping from the mold, the steam disturbs the surface of the mold so that openings are formed to admit molten metal and so that particles of molding material mingle with the metal in the surface of the solidifying casting. This problem is commonly called burning in and the resulting casting is not only difiicult to clean but the dimensions of the pattern are inaccurately reproduced.

In an attempt to avoid the burning in problem common in the green sand method of molding, various compositions have been applied to the surfaces of the mold. Such facing compositions are usually applied directly to the surface of the mold after the pattern has been removed by means of spraying, dustingor brushing. Although some of the compositions have been helpful in reducing the burning in problem, the methods of applying the compositions have disturbed the mold surfaces so that castings formed in the mold do not accurately reproduce the details of the pattern. To overcome this latter problem, the pattern is sometimes replaced in the mold after the facing has been applied to reform the mold surfaces. This requires great care and obviously is a time consuming and expensive procedure.

Generally, it is an object of this invention to provide a facing composition which may be used in the preparation of green sand molds so that castings formed in such molds have the characteristics usually attributed to castings made by dry sand molding processes and yet are as economical to produce as the green sand molds.

Another object of this invention is to provide an improved facing composition which may be used for prepar- 2 ing green sand molds to produce castings which are accurate in detail and dimensions and which require a minimum of cleaning or finishing to produce a clean smooth surface on the casting.

The improved facing composition as contemplated by the invention is a mixture of ingredients, which are pref: erably mixed in a conventional foundry mulling machine well known in the foundry art. A facing compound em: bodying the invention may be made by placing approximately 200 pounds or 66 quarts of fine silica sand and 5 quarts of bentonite into the. mulling machine. The bentonite is preferably of Western type and the sand should be of a very fine grade, that is, sand which will pass through a 140 mesh screen. For this purpose Iuniato sand or some equavalent grade of shell process grade sand is preferred.

To the mixture of sand and bentonite is added 1 quart of cereal flour such as corn flour, 1 quart of graphite and 12 quarts wood flour. The mixture of sand, bentonite, flours and graphite is then mulled in a. dry state, that is without addition of water, a sufiicient length of time. so that all of'the ingredients are uniformly distributed throughout the mixture. The period of mulling should be as short as possible since mulling for an excessive amount of time may cause the sand particles to break down to smaller sizes and render the finished facing composition objectionally impermeable to gases.

After the dry mixture of sand, bentonite, flours and graphite has been mulled the necessary length of time, approximately 3 quarts of water are added to it and, mulling is continued so that the water will be uniformly distributed throughout the mixture and the ingredients will be formed into a readily moldable plastic mass of lightly coherent granular particles. The mulling of the moistened mixture should be discontinued as soon as the desired consistency of the mixture has been attained, because continuation of the mulling for a longer period would impair the moldability of the plastic compound.

The plastic facing composition obtained in this manner contains, in percentages by volume, approximately percent fine silica sand, 1 percent of corn flour or cereal binder, 1 percent of graphite, 5 /2 percent bentonite, 13 /2 percent wood flour and 4 percent water. These percentages are illustrative of the invention and may be alfected more or less by the grades of the selected materials, but the water content should not exceed 5 percent by volume.

The bentonite used in the facing composition acts as a binder and is used for the purpose of strengthening the face of the mold. However, the quantity of bentonite necessary to obtain the desired strength is such that the free escape of gases from the mold may be obstructed. For this reason a smaller quantity of bentonite is used to obtain the benefit of the strengthening qualities of bentonite without affecting gas escape and the remainder of the required strength is provided by using a small amount of cereal binder whichdoes not appear to adversely aitect the flow of gases through the facing composition.

Graphite acts as a release agent to prevent burning in of the sand into the finished casting and thereby facilitates ready removal of the casting from the mold. The graphite also improves the flowability of the facing composition so that it more readily conforms to the shape of the pattern. In addition, the graphite facilitates removal of the pattern from the mold by acting as a lubricant to aid sliding movement between the mold and portions of the pattern.

The wood flour acts as an expansion agent so that upon pouring of the hot metal into the mold, the expansion of the sand is absorbed by the wood hour. The mold facing, therefore, is not adversely affected and a pattern-true casting results.

In utilizing the facing composition to prepare a mold, a pattern is inserted into a flask and a commercial liquid release agent may be sprayed on the surface of the pattern. The facing composition is then sifted through a riddle, preferably a number 8 riddle having 64 openings to the square inch, onto the surface of the pattern so that it is entirely covered to an average depth of approximately M1110 of an inch. The depth of the facing composition may vary in accordance with the size of the casting, but should be adequate for a suitable facing of the finished mold.

After the facing composition has been sifted to cover the pattern, surfaces, ordinary, relatively coarse foundry sand affording a backing of a high porosity or permeability is deposited on top of the facing composition to fill the flask. The porous backing permits escape of gases upon pouring of molten metal into the finished mold.

The facing composition and the backing sand in the flask are then compacted by any conventional means such as with a jolt-squeeze machine well known in the art and used in most foundries. When the pattern is removed, a mold results which is a firm, accurate impression of the pattern and which comprises a porous body of foundry sand or backing sand and a thin plastic facing integrally united with the porous body of backing sand.

, Experience has shown that castings, and particularly gray iron castings produced with molds of the herein disclosed character are closely accurate reproductions of the pattern not only as to surface contour but also as to volumetric proportions.

It has also been observed that the herein disclosed facing composition need not be used immediately after it is made since it can be stored in closed containers for relatively long periods of time, for example, two or three weeks without detracting from its desirable qualities.

It should be understood that it is not intended to limit the invention to the particular forms and details described herein and that the invention includes such other forms and modifications as are embraced by the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. A plastic facing composition for green sand molds containing substances in approximately the following proportions by volume: percent sand of a size which will pass through mesh screen, 5 /2 percent bentonite, 1 percent corn flour, 1 percent graphite, 13 /2 percent wood flour and 4 percent water.

2. In a foundry mold, the combination of a porous backing of foundry sand, and a plastic facing integrally united with said backing and consisting essentially, by volume, of approximately 75 percent silica sand of a size which will pass through a 140 mesh screen, 5 /2 percent bentonite, 1 percent corn flour, 1 percent graphite, 13 /2 percent wood flour and not more than 5 percent water.

References Cited in the file of this patent UNITED STATES PATENTS 360-366, second edition. 

1.A PLASTIC FACING COMPOSITION FOR GREEN SAND MOLDS CONTAINING SUBSTANCE IN APPROXIMATELY THE FOLLOWING PROPORTIONS BY VOLUME: 75 PERCENT SAND OF A SIZE WHICH WILL PASS THROUGH 140 MESH SCREEN, 51/2 PERCENT BENTONITE, 1 PERCENT CORN FLOUR, 1 PERCENT GRAPHITE, 131/2 PERCENT WOOD FLOUR AND 4 PERCENT WATER. 